Dipole array antennas, such as the log periodic and Yagi (or Yagi-Uda) antennas, are widely used. An attribute of the Yagi antenna is its high gain, whereas the log periodic antenna is known for its wide bandwidth. Both of these antenna types consist of at least three different length dipoles in most cases, and are primarily used for frequencies below one GHz.
The Yagi antenna typically consists of three antenna elements: a driven element of length L1 connected to an RF source and/or receiver, a director of length L2 and a reflecting element of length L3. Typically, the director length L2 is shorter than the driven element length L1 by 5%, whereas the reflector element length L3 is 5% longer than L1. The director is closely spaced in parallel to the driven element in order for radiation currents to be induced on the director's surface by near field coupling. This technique avoids the necessity of feeding multiple radiating elements individually. Higher antenna gain can be achieved by adding additional directors.
One drawback of both the log periodic and Yagi antennas is that they are not well matched to standard 50 ohm transmission lines. As a result, matching networks are required to match the antenna impedance to the 50 ohm feed line. These matching networks add to the antenna complexity and cost.
In addition, conventional log periodic and Yagi antennas are not well suited for use at higher microwave frequencies, e.g., 2.4 and 5.8 GHz Industrial, Scientific and Medical (ISM) bands. As RF communication has become more prolific at microwave frequencies, there has arisen a need for small, low cost antennas with high performance. Accordingly, the present invention addresses this need.